1. Field of the Invention
This invention relates to electronic device packaging and, in particular, to packaging for integrated circuit devices, hybrid circuits, or modules containing more than one integrated circuit chip. More particularly, the invention relates to plastic encapsulated integrated circuits in packages having leads extending from two or more package sides and incorporating an integral heat sink with an exposed outer surface.
2. Prior Art
During the early developmental stages of semiconductor packaging, the integrated circuit was typically packaged either in a metal can or between a ceramic lid and base. Both packaging materials provided excellent thermal properties, but each also necessitated expensive and time consuming packaging techniques. For example, in ceramic packages the use of two ceramic substrates reflected a sizeable percentage of the total cost of manufacturing the component.
As semiconductor production volumes grew, more cost effective packages were developed. The most notable was the plastic molded package. Although the plastic package provided significant cost savings, the advantageous thermal properties present with metal or ceramic were lacking. As integrated circuit speed and density increased, the need for improved thermal performance (i.e., improved dissipation of heat) became more important. This need motivated the inclusion of a metallic heat sink within the package to remove heat without compromising the reliability of the package because of its mechanical design and the associated assembly process.
Several attempts to improve heat dissipation have been made, the most common being incorporation of a heat sink or spreader into the package. Chu, U.S. Pat. No. 4,975,761, describes several types of heat sinks which are included inside the package. However, these methods do not provide the level of heat dissipation required by newer generations of integrated circuit assemblies.
Additionally, studies have been made of methods of bringing the surface of the heat sink to the outer edge of the package, thereby greatly reducing the thermal resistance. These attempts have been unsuccessful due to various manufacturing difficulties.
In one method, plastic molded packaging of an integrated circuit with heat sink is done by placing the assembly to be packaged in a mold cavity such that the side of the heat sink to which the semiconductor die has been attached faces into the mold cavity. The assembly is restrained by pins or a clamping mechanism attached to the leadframe assembly outside the mold cavity. The opposite side of the heat sink (the outer surface) remains exposed in the completed package. A surface of the mold cavity is brought into contact with the outer surface of the heat sink. Encapsulant is injected into the mold cavity until the cavity is full. When the encapsulant cools and solidifies, the mold cavity is opened and the completed package removed. Several problems have been encountered with this technique.
Current semiconductor package mold tooling design and manufacturing techniques have produced an insufficiently tight seal between the outer surface of the heat sink and the mold cavity. As a result, the high pressures present within the mold cavity during the encapsulation process, as well as dimensional variations from piece to piece between particular heat sinks, can result in separation between the surfaces of the heat sink and the mold cavity. This inadequate sealing allows either encapsulant bleed or flash formation on the exposed heat sink surface. Bleed is the undesirable presence of translucent encapsulant and flash is the undesirable presence of encapsulant greater in thickness than bleed and visible to the naked eye. To remedy this problem, methods were investigated which relied either on specialized mold tooling and techniques to improve the seal between the heat sink and the mold cavity during the encapsulation process, or on expensive hand flash removal after the encapsulation process is complete. Both of these remedies result in inconsistent quality and low yields.
In one method, a vacuum is pulled across the face of the heat sink, holding the heat sink against the mold cavity surface. However, if the seal is insufficiently tight, or if the outer surface of the heat sink is insufficiently flat, plastic encapsulant is sucked into the vacuum mechanism. If this happens, a difficult and expensive clean-up of the vacuum system is required, and bleed or flash results on the surface of the heat sink.
In another method, pneumatically actuated clamping pins contact the face of the heat sink facing into the mold cavity and force the face of the heat sink facing out of the mold cavity against the abutting surface of the mold cavity. However, this method requires sophisticated pin extraction techniques. These techniques are necessary to avoid interference between the pins and bonding wires or internal package leads as the pins are removed, and to avoid marring the surface of the heat sink or having the pins become stuck during retraction.
Another method involves clamping on leadframe members (the leadframe being attached to at least one side of a heat sink "paddle," another side of the paddle protruding and restrained outside of the mold cavity) external to the mold cavity and applying force to these members to push the heat sink against the mold cavity. This method is inapplicable, however, to packages with leads on four sides of the package.
All of the above techniques continue to produce various levels of encapsulant bleed and flash across the exposed surface of the heat sink. This unwanted plastic necessitates extensive and expensive cleaning of the exposed surface prior to subsequent processing operations. It also causes inferior heat dissipation.
A second problem has been encountered with plastic molded integrated circuit packages formed with a heat sink. Heat sinks in transistor package designs are known to allow various quantities of contaminants to migrate from sources outside the package to the surface of the semiconductor die and, specifically, to the bonding pads. Contaminants pass along the interface between the heat sink and the molded plastic due to the lack of an adequate seal. Penetration of the package interior results from the inability to lock the molded plastic securely to the heat sink. Though contaminants may also pass along the interface between leadframe and molded plastic, and some prior art has addressed that problem (e.g., Lehner, U.S. Pat. No. 3,564,352), the major source of contamination of the die in plastic packages with heat sinks is the open path presented by the interface between the heat sink and the molded plastic.
Some solutions to this problem, such as the use of a moisture moat, have proven to be marginally adequate remedies for older generations of less sensitive integrated circuits. However, for newer integrated circuits, these solutions are inadequate.
Other approaches have incorporated locking tabs on the sides of the heat sink. Though adequate for preventing other problems relating to transistor style packages (such as TO-220), these locking tabs are inadequate to seal out levels of moisture to which current generations of integrated circuits are sensitive.
A third problem that has been in plastic molded integrated circuit packages formed with a heat sink concerns the connections between the semiconductor die, leadframe and heat sink. Typically, these connections have been made by affixing the heat sink and semiconductor die to opposing surfaces of a leadframe die mounting paddle as shown in FIG. 6a. To enable heat transfer away from the semiconductor die, a thermally conductive adhesive is applied to paddle surfaces contacting the heat sink and die. Heat is then transferred from the semiconductor die through the first layer of adhesive to the die mounting paddle. From the die mounting paddle the heat is transferred through the second layer of adhesive, through the heat sink and then through the remaining bulk plastic material. Severe limitations have been encountered in using this technique for more thermally sensitive semiconductor designs due to the inability of the package to dissipate increased amounts of heat. Although attempts have been made to enhance the thermal flow from the die to the heat sink through the use of materials with improved thermal conductivities, the necessity for the heat to pass through the die mounting paddle and additional adhesive layer before reaching the heat sink continues to constitute a thermal bottleneck.
The connection between the leads and the heat sink presents an additional problem. The adhesive between the heat sink and leads must be electrically insulative in order to prevent shorting between the two. If this adhesive does not completely fill the space between the heat sink and leads, then undesirable electrical conduction between the heat sink and leads may occur.